Recoated spliced lengths of optical fibers

ABSTRACT

Spliced end portions (30-30) of two optical fibers are recoated in a mannerhich results in the cross section of the spliced length of fiber transverse to a longitudinal axis thereof being substantially constant. This is accomplished without compromising the adhesion of a curable recoating material (51) to an adjacent original coating material (38). In order to provide such a recoated portion, original coating material which is removed to permit splicing is removed in such a manner as to leave a tapered portion (52) remaining on the end portion of each optical fiber. As a result, the interface between the recoating material and the original coating material is increased sufficiently to avoid having to overlap some of the recoating material with original coating material on adjacent portions of the fibers being spliced. Further, the spliced, recoated end portions are positioned at the focal point of a parabolic reflective surface (75) so that a portion of the radiation which is emitted toward the spliced end portions to cure the recoating material is reflected to engage non-exposed peripheral portions of the recoating material to insure uniformity of cure.

TECHNICAL FIELD

This invention relates to recoated spliced lengths of optical fibers.More particularly, it relates to the recoating of a fusion bondedoptical fiber splice and portions of optical fibers adjacent to thesplice which provides substantially improved integrity over recoatedsplices made in accordance with the prior art.

BACKGROUND OF THE INVENTION

The use of optical communications involving the use of optical fibershas grown at an unprecedented pace. Typically, an optical fiber has adiameter on the order of 125 microns, for example, and is covered with acoating material which increases the outer diameter of the coated fiberto about 250 microns, for example Optical cables may comprise aplurality of these optical fibers which are stranded together or whichare assembled in planar arrays which are referred to as ribbon.

The technology for forming low-loss optical fibers, which is shown forexample in U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 in thenames of J. B. MacChesney and P. B. O'Connor, has advanced to a pointwhere there is widespread commercial manufacturing of optical fibers.Most processing includes drawing an optical fiber from a previouslymanufactured glass boule, sometimes referred to as a preform. After ithas been drawn, the optical fiber is usually provided with a layer of aprotective curable coating material which may be cooled or curedthermally, by radiation, or by other suitable techniques for achievingsolidification.

The introduction of optical fiber application to evermore hostileenvironments, such as in underwater cable, has required that morestringent requirements be imposed on physical properties of the fiber,such as strength. For these more demanding applications as well as forother less demanding ones, it has become increasingly more common tosplice optical fibers which have broken, either accidentally, or duringappropriate proof testing. Additionally, extremely long lengths of fibermay be obtained by splicing a plurality of lengths which are obtainedusing current manufacturing techniques. For these and otherapplications, splicing in which the coating material is removed from endportions of two fibers which are then fused together end to end providesa suitable means for joining the ends of two glass fibers with anacceptable loss. However, the recoating of bared spliced fiber endportion continues as a problem to be overcome, especially whilemaintaining stringent requirements on dimensional and strengthparameters associated with the coated fiber.

A method of recoating spiced end portions of optical fibers is disclosedin U.S. Pat. No. 4,410,561 which issued on Oct. 18, 1983 in the name ofA. C. Hart, Jr. The method involves placing the spliced fiber endportions from which the original coating material has been removed andadjacent portions within a cavity in the form of a groove in a splitmold. The effective diameter of the groove is somewhat greater than thatof the remaining coated portion of each fiber. The fibers are positionedso that only portions of the coated portions of the fibers touch thesurface which defines the groove, while the vulnerable, uncoated splicedend portions of the fibers remain suspended and do not contact thegroove surface. Then, the mold is covered to enclose the groove and asuitable curable coating material is injected into the groove to recoatthe bared, spliced fiber end portions. The recoating material contactsthe adjacent originally coated portions of the spliced fibers alongsubstantially radial planes exposed when the original coating materialwas removed from the end portions and along overlapping portions of theouter surface of the original coating material adjacent to the radialplanes. The coating material is then cured to yield a recoated splicesection with a transverse cross section which is larger than that of theoptical fiber having the original coating material thereon.

This molding process provides a recoated splice; however, steps must betaken to avoid an undesirable number of residual bubbles in therecoating material. The existence of bubbles may lead to stressconcentrations when the fiber is handled subsequently. This isparticularly undesirable in underwater cables where splices areinaccessible and under stress for many years.

It appears that there are three sources of bubbles. These are airalready present in the recoating material, air entrained during themolding process, and bubbles formed during the shrinkage of therecoating material during its cure. The bubbles due to shrinkage tend tobe concentrated at the interface between the coating on the unbaredfiber portions and the recoating material. This is caused by the pullingaway of the recoating material from the coating material on the unbaredfiber portions during curing.

Inadequate overlap between the recoating material and the originalcoating material on the unbared portions of the optical fibers isanother problem. Long term integrity of the fiber may be affected by thefailure of the recoating material to overlap adequately the originalcoating material on the portions of the fibers adjacent to the splicedend portions. It may result in the separation of the existing andrecoating materials and expose the bare fiber.

Another problem which has surfaced recently related to the use ofoptical fibers for tethered vehicles. In these, an optical fiber whichis wound on a payoff device and connected to a guidance system is payedoff as the vehicle is moved. The payoff device contains a length ofprecision wound optical fiber.

For tethered vehicles, the winding of the optical fiber on the payoffdevice must be accomplished in a precision manner. Otherwise, payoffcould be disrupted. It has been found that it is difficult to wind aprecision package with recoated splices which are made by presenttechniques. If the cross section of the recoated portion transverse ofthe longitudinal axis of the optical fiber is not the same as that ofthe optical fiber as originally coated, the winding pattern on thepayoff device in all likelihood is not uniform. This will cause problemsin fiber payoff following the launch of the tethered vehicle.

Seemingly, a recoated splice having the same transverse cross section asthat of the unspliced fiber has not been attained by the use of priorart methods and apparatus. The transverse cross section of the recoatedportion had to be larger to provide overlap of the recoating materialwith portions of the original coating material adjacent to the recoatedend portions, otherwise the necessary adhesion to the original coatingmaterial would not be achieved only along the radial planes exposed bythe baring of the end portions. When the recoated portion is made largerin a transverse cross section than that of the original coatingmaterial, a portion of the recoating material becomes adhered toperipheral portions of the original coated fiber lengths which areadjacent to the beginning of the recoated end portions of the opticalfibers and supplements the adhesion along the radial planes.

What is needed and what seemingly is not provided by the prior art is arecoated optical fiber splice which may be used in providing arelatively long length of optical fiber for use in guiding a tetheredvehicle, for example. Such a recoated splice must be implemented easily,must have the same transverse cross section as that of the originalcoated optical fiber and must have integrity of adhesion of therecoating material to the original coating material over a period oftime. Also, the sought after recoated spliced end portions of opticalfibers preferably are such that the formation of bubbles is avoidedsubstantially.

SUMMARY OF THE INVENTION

The foregoing problems of recoated prior art splices have been overcomewith the recoated splice of this invention. A first length of opticalfiber is fusion bonded to a second length of optical fiber. Prior tobonding, each length of optical fiber is prepared for splicing. This isaccomplished by chemically removing coating material from an end portionof each length of the fiber. The coating material is removed so as toleave a bared portion of optical fiber adjacent to the end of the endportion of each optical fiber and so as to leave a tapered length of theoriginal coating material extending from the bared portion to theportion of the optical fiber adjacent to the end portion. In this way,an increased length of surface is provided to become the interfacebetween the original coating material and the recoating material. As aresult, there is enhanced adhesion between the original coating materialand the recoating material, thereby insuring the integrity of therecoating material over the spliced portions of the lengths of opticalfiber. Further, this permits the recoating of the spliced end portionsto be accomplished so that the cross section of the spliced length ofoptical fiber transverse to the longitudinal axis of the optical fiberis substantially constant.

In a method of recoating spliced portions of lengths of optical fibers,the coating material on end portions of the lengths of each of twooptical fibers is removed to leave a bared portion and a portion onwhich the coating material is conically shaped. The prepared endportions which have been spliced together are positioned in a passagewayof a fixture with original coated portions of the lengths adjacent tothe end portions being held in opposed vacuum chucks at opposite ends ofthe fixture. An injection port communicates a supply of a curablerecoating material with the passageway. The passageway is formed in amold block of material which is transparent to radiation used to curethe recoating material. The mold block of material is supported on ametallic pedestal. Along an upper surface of the pedestal is provided atrough which is in alignment with the passageway and spaced therefromand which includes a polished reflective surface.

The methods and apparatus used to provide the recoated spliced lengthsof optical fibers of this invention assure that the recoating materialwhich covers the spliced portions of the lengths of optical fibers arecured uniformly. After a curable recoating material has been injectedthrough the port into the passageway and caused to encapsulate the baredand tapered portions of the end portions, it is cured by exposing it tosuitable radiation. The trough in the metallic pedestal has a transversecros section which is parabolic. Advantageously, the passageway ispositioned at the focal point of the parabolic cross section of thetrough so that radiation extending past the optical fiber is reflectedby the wall of the trough and caused to engage the peripheral portion ofthe optical fiber which is not exposed directly to the radiation. As aresult, the entire periphery of the optical fiber portions in thepassageway is cured uniformly.

BRIEF DESCRIPTION OF THE DRAWING

Other objects and features of the present invention will be more readilyunderstood from the following detailed description of specificembodiments thereof when read in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a perspective view of spliced end portions of two lengths ofoptical fibers which have been prepared in accordance with thisinvention to receive a recoating material;

FIG. 2 is a perspective view of end portions of lengths of optical fiberwhich have been prepared for recoating in accordance with prior artmethods and apparatus;

FIG. 3 is a side elevational view of one of the end portions of FIG. 2after a recoating material has been applied thereover;

FIG. 4 is a side elevational view of one of the spliced end portions ofFIG. 1 after a recoating material has been applied thereover;

FIG. 5 is a schematic view of an apparatus for removing coating materialfrom a portion of the length of an optical fiber;

FIG. 6 is a perspective view of an apparatus for recoating splice endportions of two lengths of optical fibers;

FIG. 7 is a perspective view of a fixture of the apparatus of FIG. 6which may be used to support spliced end portions of lengths of opticalfibers for recoating in accordance with this invention;

FIG. 8 is a perspective view of a prior art fixture for supportingspliced end portions of optical fibers to be recoated;

FIG. 9 is a perspective view of spliced lengths of optical fiber woundon a spool, the spliced portion of which has been recoated by prior artmethods; and

FIG. 10 is a perspective view of spliced lengths of optical fiber woundon a spool, the spliced portion shown having been recoated in accordancewith the methods described herein.

DETAILED DESCRIPTION

Referring now to FIG. 1, there are shown end portions 30--30 of twolengths, a first length 32 and a second length 33, of optical fiberwhich have been spliced together to form a juncture 35. Each of theoptical fibers includes an optical fiber 36 having a coating material 38applied thereon (see FIG. 1). As is well known, the optical fiber 36includes a core and a cladding. An outer diameter of the coated opticalfiber is on the order of 250 microns. The bared end portions 30--30 fromwhich the coating material 38 has been removed at least partially havebeen spliced together by a technique such as fusion bonding which isdisclosed in an article entitled "Optical Fiber Joining Technique" whichwas authored by D. L. Bisbee and which appeared beginning at page 3153of Vol. 50 No. 10 of the December 1971 issue of the Bell SystemTechnical Journal. Each end portion 30 has a length of about 0.5 inch.The spliced end portions 30--30 are recoated in accordance with themethod and apparatus of this invention.

Typically, spliced end portions of optical fiber lengths prior torecoating appear as shown in FIG. 2. There it can be seen that a coatingmaterial 40 has been removed from an end portion 42 of each of twooptical fibers 44 and 45 which have been spliced together to form ajuncture 46. The portion of the coating material that has been removedis generally in the shape of a cylindrical tube or sleeve. An exposedend face 47 of the coating material lies generally in a plane whichgenerally is normal to a longitudinal axis 48 of the optical fiber (seeFIG. 3). After the end portions of the optical fiber lengths have beenfusion bonded together, they must be recoated. This may be accomplishedby using methods and apparatus such as, for example, those disclosed inU.S. Pat. No. 4,627,942 which issued on Dec. 9, 1986 in the names of P.F. Gagen, et al. and which is incorporated by reference hereinto.Typically, a portion 49 of a recoating material extends past the radialend face plane 47 of the end portion 42 of the optical fiber andoverlaps a portion of the original coating material (see FIG. 3). Thisis done in order to provide sufficient interfacial area between theoriginal coating material and the recoating material to insure theintegrity of the coating and recoating materials along the interfacetherebetween over a period of time.

As can best be seen in FIG. 3, the interface between the recoatingmaterial 49 and the original coating material without the overlap is notmuch greater than the cross-sectional annular area of the originalcoating material. As a result, the opportunity for adhesion between theoriginal coating and recoating materials is somewhat limited. Of course,the overlapped portion is additive to this, but this causes the crosssection of the recoated end portion of the optical fiber transverse tothe longitudinal axis of the optical fiber to exceed that of the opticalfiber with the original coating material thereon.

This problem is overcome by configuring the stripped end portion of eachlength of optical fiber to appear as is shown in FIGS. 1 and 4. As canbe seen in those figures, the coating material 38 is removed completelyfrom a portion 50 of each end portion 30. Also, a tubular portion of thecoating material is removed from each end portion 30 adjacent to thebared portion 50 in such a manner as to cause a tapered portion 52 toremain. The tapered portion 52 of original coating material has agenerally truncated conical shape. In other words, in the vicinity ofthe splice juncture 35, the original coating material increases from atransverse cross section substantially equal to the transverse crosssection of the uncoated clad optical fiber of the bared portion 50 tothe transverse cross section of the original coated optical fiber.Typically, the distance from the juncture 35 to the beginning of thetapered portion 52, i.e. the length of the bared portion 50, is about0.25 inch or more and the length of the tapered portion of the originalcoating material is equal at least to the product of three and thelargest dimension of a transverse cross section of a coated opticalfiber but does not exceed a value of about 0.25 inch.

Such a configuration of the coating material remaining after strippingis highly advantageous during recoating. As can best be seen in FIG. 4,a recoating material 51 abuts the original coating material along aninterface which is substantially larger than that of FIG. 3. As aresult, there is substantial interfacial contact between the originalcoating material 38 and the recoating material 51 to provide sufficientadhesion and prevent unwanted separation of the recoating material fromthe spliced end portions of the lengths of the optical fibers.

Further, and importantly, the increased interfacial contact of therecoating material 51 and the original coating material 38 obviates theneed for overlap as used in FIG. 3. As a result, the transverse crosssection of the recoated end portions 30--30 may be held to besubstantially the same as that of the optical fiber having the originalcoating material 38 thereon. In other words, the cross section of thecoated optical fiber along portions having the original coating material38 thereon and along those portions which have been recoated issubstantially constant.

Prior to the fusion bonding and recoating steps of providing a splicedlength of optical fiber, the end portions 30--30 of two lengths ofoptical fiber must be prepared. Preparation includes the completeremoval of the original coating material from a portion 50 of the endportion 30. Preparation also includes the removal of a portion which iscontiguous to the portion 50 (see FIG. 1) and which has a transversecross section that decreases in a direction from the portion 50 to thecoated optical fiber adjacent to the end portion 30.

In FIG. 5, there is shown a holder 53 for holding an optical fiberlength 32 above a container 55 of a suitable liquid material such as anacid which may be used to remove coating material from an end portion 30of an optical fiber to provide the configuration shown in FIG. 1. Theholder 53 is adapted to be moved vertically reciprocally by turning acam 57.

An operator mounts the optical fiber 32 in the holder 53 with a portionof the optical fiber contiguous to an end of the optical fiber beingimmersed in the liquid material in the container 55. This causes thecoating material 38 to be removed completely from that portion which isimmersed to form a bared portion. Then the operator causes the cam 57 tobe turned. As a result, the portion of the end portion 30 of the opticalfiber which is adjacent to the bared portion is moved into and out ofthe container 55 in one or more cycles to remove the coating materialpartially therefrom to form the tapered portion 52 shown in FIG. 1. Theconfiguration of the tapered portion 52 is a function of the temperatureof the liquid material in the container 55 as well as the cycle time andthe configuration of the cam. Subsequently, the operator causes thebared portion to be broken to provide the bared portion 50 having adesired length.

Then, end portions of two lengths of optical fibers having the coatingmaterial thereon removed as described hereinabove are spliced togetherby fusion bonding for example. The spliced end portions 30--30 are nowready to be recoated.

Shown in FIG. 6 is an apparatus designated generally by the numeral 60for recoating the spliced together ends of the lengths of opticalfibers. The apparatus 60 includes a base 62 which includes two ways 64and 66 spaced along a longitudinal axis of the base. Mounted in each ofthe ways 64 and 66 is a vacuum chuck 68. Each chuck 68 includes a groove71 which extends generally parallel to the longitudinal axis of thebase. Each of the grooves 71--71 has a transverse cross section suchthat it is capable of holding a length of coated optical fiber. Further,the wall of each of the grooves 71--71 is connected to a source ofvacuum (not shown) to hold a coated optical fiber in the groove duringthe recoating process.

As further can be seen in FIG. 6 (see also FIG. 7), a center portion 73of the base 62 is raised somewhat over adjacent portions. The centerportion 73 includes a longitudinally extending trough 75 which intransverse cross section has a parabolic configuration. This trough 75is referred to as a reflective chamber and typically is milled in a basewhich is made of aluminum. The surface of the trough 75 is polished.

Supported on the center portion 73 is a mold block 80 (see FIGS. 6 and7). The mold block 80 includes a longitudinal extending groove 82 whichis adapted to hold the spliced end portions of two lengths of opticalfibers. Projecting upwardly from two corners of the mold block 80 aretwo alignment pins 84--84. The alignment pins 84--84 are used to align atop mold block 90 with the mold block 80. The mold blocks are made froma material which is transparent to ultraviolet (UV) radiation such asPlexiglas® UV transparent material or equivalent resin material orquartz, for example.

The top mold block 90 also includes a longitudinally extending groove 92which is adapted to cooperate with the groove 82 in the mold block 80 toprovide a passageway 93 to enclose the spliced end portions of the twolengths of optical fibers. Further, the top mold block 90 is secured tothe mold block 80 to provide a close fitting groove for those endportions by four bolts 94--94, or other suitable clamping means.

When the top mold block 90 is secured to the bottom mold block 80,passageway 93 communicates with an injection nozzle 96 (see FIG. 6). Theinjection nozzle 96 is connected to a supply of recoating material whichpreferably is the same material which was used to coat the drawn fiber.Such a material may be a UV curable acrylate material, for example.

Portions of the spliced lengths 32 and 33 of the optical fibers adjacentto the end portions 30--30 are caused to be received in the grooves71--71 in the chucks 68--68. Vacuum is applied to hold those portionsduring the recoating step. With the portions of the lengths of opticalfibers held in the vacuum chucks 68--68, the lengths of the opticalfiber which are spliced together are disposed in the passageway 93formed by the cooperating grooves in the top mold block 90 and the moldblock 80.

Also, it should be observed that the passageway 93 formed by the groovesin the top mold 90 and in the mold 80 is aligned with the reflectivechamber 75. The spatial relationship of the passageway 93 and thereflective chamber 75 is such that the passage is disposed at the focalpoint of the parabolic configuration of the reflective chamber. As aresult of this arrangement, the curing of the recoating material byexposure to UV radiation from a source (not shown) is enhancedsubstantially. This will become apparent by comparing FIGS. 7 and 8. InFIG. 8, there is depicted a prior art apparatus 100 for curing therecoating applied over spliced end portions. As the UV radiation isdirected past the spliced end portions, it engages a metallic base plate99 and is reflected. It is unlikely that the reflected radiation will bedirected to the underside of the recoating material. Hence, therecoating material may not be cured uniformly.

On the other hand, the arrangement which is shown in FIG. 7 assuressubstantially uniformity of cure in the recoating material. The UVenergy radiating past the optical fiber end portions 30--30 andcontacting the parabolic surface is reflected. Because the recoatedportions are disposed at the focal point of the parabolic reflectingsurface, the reflected radiation contacts those portions of theperiphery of the recoating material which are not exposed directly tothe emitted radiation, thereby ensuring the uniformity of cure.

The resulting product is a relatively long length of optical fiber whichhas a substantially constant cross section transverse to thelongitudinal axis of the optical fiber. The cross section of therecoated spliced portions transverse to the longitudinal axis of theoptical fiber are substantially equal to that of the unspliced endportions. This equivalence of transverse cross section is advantageousparticularly in the winding of the fiber on a spool or other payoutdevice. With prior art recoatings, as mentioned hereinbefore, thetransverse cross section of a spliced portion is larger than that of theunspliced potions. As a result, when such spliced portions are woundwith convolutions of optical fiber on a spool laterally and outwarldy,as shown by the arrows 101 and 102 in FIG. 9, the winding pattern isdisrupted and non-precise winding occurs. The overlapped portion 49 (seeFIGS. 3 and 9) of a recoating material causes unwanted displacement inthe direction shown by the arrow 101 in FIG. 9 of adjacent convolutionsin the same layer and of those conductors above the spliced portioncovering the spliced portion. As a result, those portions of theconvolutions are not nexted between adjacent convolutions of adjacentlayers. Furthermore, the enlarged splice causes unwanted bulging of theportion of fiber directly covering the splice, as can be seen byportions 104, 105, 106, and 107, for example in FIG. 9. This may resultin snags during payout as portions of outer convolutions become caughtin spacing between the spliced portions and convolutions adjacentthereto. With spliced portions recoated by the methods and apparatus ofthis invention, a precise winding pattern is achievable without anypacing between convolutions which include the recoated portions andthose that do not (see FIG. 10).

The recoating technique described herein also helps to avoid theoccurrence of bubbles adjacent to the interface. Bubbles tend to becomeentrapped at the interface between the original coating material and therecoating material. Also, when the recoating material is applied, itcontracts and tends to pull bubbles outwardly from the original coatingmaterial into the interface. The existence of bubbles is unwanted,particularly at the interface, because of possible adverse affects onthe adhesion level across the interface. It has been found that becauseof the lengthened interface provided by the methods and apparatus ofthis invention, any bubbles tend to be moved outwardly toward the outersurface of the optical fiber and are not residual in the recoated spliceportions.

It is to be understood that the above-described arrangements are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

What is claimed is:
 1. A spliced length of optical fiber, said splicedlength of optical fiber comprising:a first length of optical fibercomprising an optical fiber which is provided with an outer layer of acoating material having a constant cross section transverse to alongitudinal axis of the spliced length of optical fiber and whichincludes an end portion along at least a portion of which the opticalfiber is provided with coating material having an outer surface that istapered with respect to the longitudinal axis; a second length ofoptical fiber comprising an optical fiber which is provided with anouter layer of a coating material having the constant cross section andwhich includes an end portion along at least a portion of which theoptical fiber is provided with coating material having an outer surfacethat is tapered with respect to the longitudinal axis, said secondlength having the end portion thereof spliced to the end portion of saidfirst length of optical fiber to form a juncture between ends of theoptical fibers; and a recoating material which is disposed about saidjuncture and which engages the tapered coating material on each endportion along substantially an entire generally conically shapedinterface with the coating material, said interface extending from theoptical fiber of said each end portion to the coated optical fiberhaving the constant cross section adjacent to said each end portion insuch a manner that the cross section of the spliced length of opticalfiber transverse to the longitudinal axis is substantially uniform andsubstantially equal to that of the coated optical fiber having theconstant cross section.
 2. The spliced length of optical fiber of claim1, wherein said recoating material is the same material as originalcoating material.
 3. The spliced length of optical fiber of claim 1,wherein the interface of the recoating material with the coatingmaterial extends a distance which is at least about three times thediameter of the coating optical fiber, but which does not exceed a valueof about 0.25 inch.
 4. The spliced length of optical fiber of claim 1,wherein each end portion of each length of optical fiber includes acylindrically shaped tubular portion of recoating material which engagesthe otical fiber of the end portion and which is contiguous to saidjuncture and a tapered portion which extends between said cylindricallyshaped tubular portion and the coated optical fiber contiguous to saidend portion, said tapered portion having an annular cross section whichdecreases in a direction from said cylindrically shaped tubular portionto the coated optical fiber.